1. Field of the Invention
This invention pertains to apparatus and an accompanying method for a jack assembly that secures a grip and specimen held thereby into proper position in a jaw of a materials testing machine and applies suitable pressure and restraint to the grip during mechanical testing. The present invention is well suited for use in a variety of different mechanical and thermo-mechanical material testing machines, including those which provide compressive and tensile testing of a mechanical test specimen.
2. Description of the Prior Art
Conventional materials testing machines utilize a wide range of screw jacks, wedge plates and other types of jack assemblies in order to secure a grip, as it holds an end of a specimen under test, in proper position in a jaw during a mechanical test.
In such a machine, typically a known and controlled compressive and/or tensile force is applied, often under computer control and as defined by a so-called “test program”, to one of two grips that hold the specimen and thus deform the specimen under predefined test conditions. Often, a test program may encompass applying a series of such forces—tensile and/or compressive, i.e., commonly referred to “hits”, to the specimen to increasingly deform it. Dilation and/or other physical measurements are typically made of the amount of resulting deformation as or after each hit is made to the specimen. In a thermo-mechanical material test system, the specimen may be controllably and self-resistively heated, such as through serial passage of electrical heating current through the jaws and specimen, before, simultaneously with or after each such hit and as defined in the test program. Such systems are exemplified by the GLEEBLE dynamic thermo-mechanical material test systems presently manufactured by Dynamic Systems, Inc. of Troy, N.Y., which is the present assignee (GLEEBLE being a registered trademark of Dynamic Systems, Inc.).
Generally speaking, cams have been previously used to apply pressure to a component in a variety of machines. However, cams have traditionally not been used as a component of jack assemblies used in materials testing systems for the simple reason that a cam has a tendency to loosen unless the cam is jacked past its center position, i.e., rotated “over-center”. In such a testing system, a mechanical force, generated through, e.g., a rod of an external servo-controlled hydraulic piston attached to one of the grips, the other grip often being secured in a fixed position, must be consistently applied up to a peak value through the one jaw and grip to the specimen end to produce each hit. If a cam were to be used in a jack assembly to properly hold that one grip in its corresponding jaw, a problem arises in that once the cam were to be rotated over-center to lock its grip in place, a mechanical force applied by that cam alone onto that grip decreases from its peak value that arises at a top dead center position of the cam. Consequently, once the cam would be turned over-center, that force applied by the cam decreases from its peak value applied at the top dead center position of the cam, thus reducing the lock force holding the grip in position during each subsequent hit, thus potentially allowing some specimen movement which, in turn, might inject some unwanted artifact into the resulting amount of deformation arising from that hit. Such a result may be problematic. Further, the externally generated forces required to deform the specimen during each hit are rather large and must be transmitted through the jack assembly and maintained on the specimen without the jack loosening throughout any of the hits that occur during an entire test program.
U.S. Pat. No. 3,403,549 (issued to A. G. Griffen on Oct. 1, 1968) appears to disclose the use of a cam as a component of a grip used in a materials testing machine. There, the cam acts as a pivot pin for a lever arm for moving jaw members backward in order to open two wedge grips to insert a test specimen therebetween. The cam is apparently not used to apply pressure in order to lock the jaw members into a closed position to grip the specimen.
Nevertheless, using a cam in a jack assembly would be advantageous because a resulting assembly would be capable of generating more force than would a similarly-sized screw-type jack assembly. In that regard, if a user were to manually apply torque to a cam-based jack assembly to generate a given mechanical force to lock the cam into position, a significantly higher amount of torque would need to be applied to the screw-type jack to generate the same amount of force. Unfortunately, a screw that could withstand that much torque would likely be too large to comfortably fit within a jack assembly used in a materials testing system, and hence, in that case, would render a screw-type jack assembly impractical.
Consequently, a yet unmet need exists in the art for a cam-based jack assembly, particularly suited for use in a materials testing system, that will rigidly hold a specimen grip in position in a jaw housing during entire test programs but which does not need to be jacked over-center.